The following U.S. patents are believed to represent the current state of the art:
U.S. Pat. Nos. 5,508,052; 5,000,967; 4,284,651; 4,530,278; 5,178,057; 5,088,393; 5,286,508; 5,817,360; 6,220,153 and 6,371,014.